15% is still 4% less than the UK, so there likely is no fear of losing tax base to them.
Well, the UK is unappealing for businesses right now for other reasons, but the main argument against raising the tax rate is that it reduces the difference with other EU nations that can compete better in other areas. If most of your clients and suppliers are in Paris, Amsterdam and Frankfurt, doing business from Dublin incurs various costs and inconveniences — overseas shipping of supplies and products, business travel for meetings, and enticing potential recruits from the continent to relocate to a distant island with a chronic housing shortage, to name a few.
So, those who support a low tax rate in Ireland argue that the tax rate should not only be low, but low
enough that the tax savings offset these costs. I'm not an economist and I can't predict what kind of impact raising the corporate tax rate will have, I just hope it works out for the best.